Vacuum deposition is a common method for coating metals such as aluminum, copper, zinc, and tin onto various substrates of metal, glass and plastic. The metal is vaporized by means of electric resistance heating in a vessel commonly referred to as an "evaporator heater" or a "vaporization boat". The vessel is connected to a source of electrical power to heat the vessel to a temperature which will cause the metal charge in contact with the boat to vaporize. Usually, the metal is vaporized in an evacuated chamber in which the product to be coated is placed. The product may be individually introduced into the evacuated chamber, or continuously fed through the chamber or, alternatively, may form part of the chamber itself. A wide variety of product is coated with metal using vapor deposition including, e.g., television picture tubes, automobile headlights, toys and the like.
The metal charge is placed into a recessed cavity machined into the top surface of the vessel. Upon reaching the vaporization temperature of the metal the metal charge melts and quickly vaporizes. The charge flashes in a vigorous and abrupt manner and the process is repeated with a new charge of metal. The number of flashes which can be performed in a given time period controls product production. Accordingly, the number of flashes a vessel can sustain without failure is a critical performance characteristic of the process. Conversely, when failure occurs the parts being coated are scrapped or have to be redone which involves additional time and cost. To avoid this extra cost the vessel may be scrapped before its anticipated failure. The shorter the life of the vaporization vessel the higher the production costs per part which translates into reduced efficiency.
A flash evaporator heater is formed from a substrate which is typically composed from an intermetallic composite or from graphite and may be coated with an outer coating of pyrolytic boron nitride "PBN" or pyrolytic graphite "PG". The outer coating is usually formed by a conventional chemical deposition process. The substrate is generally of rectangular configuation with the recessed cavity machined into the substrate to form a lateral cross sectional geometry of hemispherical shape and a longitudinal cross sectional geometry, the ends of which are also of hemispherical shape. The hemispherical shaped cross sectional geometry of the cavity is consistent with the simple one step machining operation used in forming the cavity in the least expensive manner.
It has been discovered in accordance with the present invention that when the substrate is coated with PBN or PG the cross sectional geometry at the longitudinal ends of the cavity develop thermal stresses due to the relative length changes between the substrate and the coating during thermal cycling. It has been discovered that such thermal stress can be relieved by reshaping the longitudinal ends to form a more gradual transition. In a companion application U.S. Ser. No. 08/166470 filed by Applicant on Dec. 14, 1993 and entitled Boron Nitride Vaporization Vessel it was similarly discovered that by modifying the lateral cross sectional geometry of the cavity at the intersection with the top surface of the vessel to form an included angle with the top surface of greater than 90 degrees and preferably over 120 degrees the useful life of the vessel would be improved. Although this may inherently relieve some thermal stress it does not solve the major stress problem occuring from the existence of a differential coefficient of themal expansion "CTE" between the coating and the substrate. Such thermal stresses are greatest in the direction parallel to the greatest dimension where there is the greatest differential movement. This is obviously along its length.